Why does this program crash, if SPLIT is between 4 and 7 - c

Question

I had a task and the program is working, for the most part, however, it crashes if I put SPLIT value between 4 and 7 (crashes at different values, if I change SIZE, but for sake of simplicity, let's keep it at 10).
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#include<string.h>
#define SIZE 10
#define SPLIT 4
#define LOW 0
#define HIGH 10
void generateArray(int data[],int size,int low, int high){
srand(time(NULL));
for(int i=0;i<size;++i){
data[i]=rand()%(high-low+1)+low;
}
}
int splitData(int arraySize, int startArray[], int splitPoint, int **firstNewArray, int **secondNewArray){
if(arraySize < 1){
return -1;
}
if(splitPoint < 1 || (splitPoint >= arraySize)){
return -1;
}
if(*firstNewArray != NULL || *secondNewArray != NULL){
return -1;
}
*firstNewArray = malloc(splitPoint * sizeof(int));
*secondNewArray = malloc((arraySize - splitPoint) * sizeof(int));
for(int i = 0; i < arraySize; ++i){
if(i < splitPoint){
(*firstNewArray)[i] = startArray[i];
printf("%d\n",startArray[i]);
}else{
(*secondNewArray)[i] = startArray[i];
printf("%d\n",startArray[i]);
}
}
return 0;
}
int main(){
int arraySize = SIZE ;
int *startArray = malloc(arraySize * sizeof(int));
generateArray(startArray,arraySize,LOW,HIGH);
int splitPoint = SPLIT;
int *firstNewArray = NULL;
int *secondNewArray = NULL;
int result;
result = splitData(arraySize, startArray, splitPoint, &firstNewArray, &secondNewArray);
if(result == 0){
for(int i = 0; i < arraySize; ++i){
if(i < splitPoint){
printf("First array number %d is %d\n",i+1,firstNewArray[i]);
}else{
printf("Second array number %d is %d\n",i,secondNewArray[i]);
}
}
free(firstNewArray);
free(secondNewArray);
}
free(startArray);
return 0;
}
What could be the cause of this behavior and how could I fix it? The task is to split startArray by the value SPLIT into 2 new dynamic arrays, that would be created in a function splitData and both of them could be used outside the function.

Answer 1

You have two issues with your code
first when you display the results:
for(int i = 0; i < arraySize; ++i){
if(i < splitPoint){
printf("First array number %d is %d\n",i+1,firstNewArray[i]);
}else{
printf("Second array number %d is %d\n",i,secondNewArray[i]);
}
}
This will not work specialy if array size is too higth or too low, example splitPoint is 9, this means secondNewArray Size is 1 but in this loop you are accessing secondNewArray[9] where it should be 0, you need to change the loop into something like this
for(int i = 0; i < splitPoint; ++i){
printf("First array number %d is %d\n",i+1,firstNewArray[i]);
}
for(int i = 0; i < SIZE - splitPoint; ++i){
printf("Second array number %d is %d\n",i+splitPoint+1 ,secondNewArray[i]);
}
You have the same isssue in your split function:
for(int i = 0; i < arraySize; ++i){
if(i < splitPoint){
(*firstNewArray)[i] = startArray[i];
printf("%d\n",startArray[i]);
}else{
(*secondNewArray)[i] = startArray[i];
printf("%d\n",startArray[i]);
}
}
In this case also you are accessing regions outside the size of your array, let say split is 9 you will be accessing secondNewArray[9] = startArray[9] where it should be secondNewArray[0] = startArray[9], to fix this you need to do the same thing here where you use different index for each array, the code should look like this:
int j = 0;
int k = 0;
for(int i = 0; i < arraySize; ++i){
if(i < splitPoint) {
(*firstNewArray)[j] = startArray[i];
printf("%d\n",startArray[i]);
j++;
}
else {
(*secondNewArray)[k] = startArray[i];
printf("%d\n",startArray[i]);
k++;
}
}

Answer 2

Take a hard look at the marked line below
for(int i = 0; i < arraySize; ++i){
if(i < splitPoint){
(*firstNewArray)[i] = startArray[i];
printf("%d\n",startArray[i]);
}else{
(*secondNewArray)[i] = startArray[i]; // LOOK HERE
printf("%d\n",startArray[i]);
}
Assuming an array size of 10 and a split point of 4, then *secondNewArray is indexed from 0 to 5; however, you’re trying to assign elements 4 through 9, which is outside the bounds of the array, leading to undefined behavior. You need to adjust the value of i in order to map properly:
(*secondNewArray)[i - splitPoint] = startArray[i];